Properties of the Feldspars. Ill 



Ab x An v — With this member of the feldspar group a diffi- 

 culty in effecting crystallization in the molten mass becomes 

 noticeable. Undercooling will continue until the vitreous melt 

 becomes rigid, unless the cooling is slow or some special effort 

 in the way of mechanical disturbance or the introduction of 

 nuclei is applied. Furthermore, when once precipitated, crys- 

 tal formation goes on slowly, even when the charge is finely 

 powdered, and the crystals are always small. Of the feldspars 

 at least it is possible to say that the k size of indi vidua] crystals 

 varied chiefly with the viscosity ; the thinner, calcic feldspars 

 always gave large individuals, while AbjAn^ Ab 2 An 15 AbgAUj 

 and Ab 4 Anj crystallized in closely interwoven, increasingly 

 smaller fibers, which gave much difficulty in microscopic 

 study. In comparison with this apparent effect of the viscos- 

 ity, the rate of cooling was altogether insignificant in deter- 

 mining the size of individual crystals. 



Several days were required to complete the crystallization of 

 100 grams of Ab ] An 1 under the most favorable conditions 

 which we were able to bring to bear upon it. The melting 

 temperature of the crystalline feldspar was still fairly well 

 marked, however, and crystallization began in the powdered 

 vitreous material as low as 700°. The melting point of this 

 feldspar is : 











ABjAN]. 









E. M. F. 



Tem- 





Date. 



Element. 



in MV. 



perature. Eemarks. 



'eb 



. 9, 1904 A 



14402 



1416° covered charge, heating rapid 



u 



9, 



A 



14400 



1416 



a 



10, 



'• No. 3 



14529 



1421 very rapid. 



it 



12, 



" 2 



14572 



1415 



a 



27, 



" 2 



14709 



J 426 very small charge. 



Mean 1419° 



Melting temperature 1419°. 



Ab^An^ — To effect the complete crystallization of this sub- 

 stance, it is best to reduce it to a fine powder and heat very 

 slowly, holding the temperature for many days at 100-200° 

 below the melting point. When thoroughly crystallized, it has 

 a melting temperature which is determinable with reasonable 

 certainty, but neither this nor any of its thermal phenomena 

 approach the more calcic feldspars in sharpness. For this 

 reason a considerably greater variation will be noticed in the 

 melting points tabulated below. 



